The present invention relates to computed tomography (CT) imaging apparatus; and more particularly, contrast-enhanced images acquired at more than one x-ray energy level.
In a current computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the “imaging plane.” The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile.
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a “view” and a “scan” of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a display.
Dual source CT systems have two separate x-ray sources and associated detector arrays, which rotate together in the gantry during a scan. The x-ray sources may be operated at different energy levels to acquire two image data sets from which a low energy and a high energy image may be reconstructed.
Proper calibration of image pixel values versus x-ray beam energy can be used for qualitative and quantitative material composition evaluation of a scanned object. The ratio of CT numbers at high and low energies can be calculated for a given material and later be used to selectively remove this material from the image. The ratio is calculated using mean pixel value over the area of the material being removed. For this reason, the subtraction of a material leaves residual pixels with comparatively low values. Such a procedure is called “dual energy image subtraction” and the result of using a conventional method is shown in FIG. 3. In this conventional method pixel values of two corresponding images acquired at different x-ray beam energies are subtracted from each other using a ratio calculated for one particular material. Thus, the particular material, or tissue type, is removed from the subtracted images, but the resulting image has poor quality and severely degraded intensity levels over the entire image. In addition, the brightness in the final subtraction images is no longer measured in Hounsfield units.